One of the most widely used biocompatible polymers for medical use is hyaluronic acid. It is a naturally occurring polysaccharide belonging to the group of glycosaminoglycans (GAGs). Hyaluronic acid and the other GAGs are negatively charged heteropolysaccharide chains which have a capacity to absorb large amounts of water. Hyaluronic acid and products derived from hyaluronic acid are widely used in the biomedical and cosmetic fields, for instance during viscosurgery and as a dermal filler.
Water-absorbing gels, or hydrogels, are widely used in the biomedical field. They are generally prepared by chemical crosslinking of polymers to infinite networks. While native hyaluronic acid and certain crosslinked hyaluronic acid products absorb water until they are completely dissolved, crosslinked hyaluronic acid gels typically absorb a certain amount of water until they are saturated, i.e. they have a finite liquid retention capacity, or swelling degree.
Since hyaluronic acid is present with identical chemical structure except for its molecular mass in most living organisms, it gives a minimum of reactions and allows for advanced medical uses. Crosslinking and/or other modifications of the hyaluronic acid molecule is necessary to improve its duration in vivo. Furthermore, such modifications affect the liquid retention capacity of the hyaluronic acid molecule. As a consequence thereof, hyaluronic acid has been the subject of many modification attempts.
Hyaluronic acid products for injection are often combined with a suitable anaesthetic, e.g. lidocaine, to reduce pain or discomfort experienced by the patient due to the injection procedure.